Flatten proxy config to a bool (drop the caddy key)

CaddySpec had only `enable` left (path_prefix/host removed; extra_snippets was
unused), so `proxy: { caddy: {} }` was three levels of nesting for a checkbox —
and it invited drift (supabase silently reverted to proxy.caddy.host). Replace it
with a plain `ServiceSpec.proxy: bool`.

- Model: remove ProxySpec/CaddySpec; `proxy: bool = False`. Readers simplified to
  `bool(svc.proxy)` (generator, castle-api summaries, CLI info).
- CLI create / dashboard editor + create form: write `proxy: true` / a checkbox.
- Configs migrated to `proxy: true` (and supabase's stale proxy.caddy.host block
  removed).
- Docs (registry, dns-and-tls, design, stack guides) + tests/fixtures updated;
  also fixed lingering path-model staleness in the stack guides.

cli/src/castle_cli/commands/create.py

@@ -8,14 +8,12 @@ import subprocess
 from castle_cli.config import REPOS_DIR, load_config, save_config
 from castle_cli.manifest import (
     BuildSpec,
-    CaddySpec,
     DefaultsSpec,
     ExposeSpec,
     HttpExposeSpec,
     HttpInternal,
     ManageSpec,
     ProgramSpec,
-    ProxySpec,
     RunPython,
     ServiceSpec,
     SystemdSpec,
@@ -121,7 +119,7 @@ def run_create(args: argparse.Namespace) -> int:
                     health_path="/health",
                 )
             ),
-            proxy=ProxySpec(caddy=CaddySpec()),  # expose at <name>.<gateway.domain>
+            proxy=True,  # expose at <name>.<gateway.domain>
             manage=ManageSpec(systemd=SystemdSpec()),
             # python-fastapi scaffold reads env_prefix MY_SERVICE_ — map castle's
             # computed port/data dir to those vars (explicit, no hidden injection).